Household power generally has a voltage of 85-264VAC. A Cold Cathode Fluorescent Lamp (CCFL), which is used as a discharge lamp for a backlight of a general LCD monitor, requires a voltage much higher than the household voltage. On the other hand, all display circuits of the general LCD monitor such as a video control circuit, other than the CCFL, use a DC voltage lower than the household voltage. For example, multi-lamp LCD monitors require a rated voltage of 12-15VDC, whereas CCFLs require a voltage higher than about 1,000VAC for lighting and a voltage of 500-700VAC for discharging.
To meet these requirements, as shown in FIG. 1, the conventional power supply system allows an AC input from a socket to pass through a rectifier/filter 1, a fly-back converter 2, a DC-AC inverter 3, and a buck regulator 4, so that AC power is supplied to CCFLs and DC power is supplied to display circuits. The conventional power supply system performs conversion between AC and DC at too many stages, thereby causing inconvenience and inefficiency. Specifically, the rectifier/filter 1 and the fly-back converter 2 are integrated into an additional adapter, which is connected to the DC-AC inverter 3 and the buck regulator 4 through an additional connector (not shown) and an additional cable (not shown). This reduces the power efficiency of the conventional system to about 70% and increases the manufacturing costs and size thereof. In addition, a conventional ferrite step-down transformer (not shown) used in the DC-AC inverter 3 is not only combustible but also causes Electromagnetic Interference (EMI) noise.
A piezoelectric transformer has been developed to overcome these problems. The piezoelectric transformer has a variety of advantages such as a high power efficiency of about 98%, low EMI noise, incombustibility, and simple CCFL operation control. The piezoelectric transformer is a vibrator that includes a piezoelectric body and two pairs of input and output electrodes formed on the surfaces of the piezoelectric body and that converts an electrical input signal to a mechanical signal, thereby mechanically transferring electrical energy. The input and output electrodes are arranged to provide impedance transformation, thereby achieving voltage transformation. The output voltage of the piezoelectric transformer depends on its operating frequency and load impedance. All CCFL operations including ignition (with a very high impedance load) and preset current control can be simply controlled by changing the switching frequency near the resonance frequency with a maximum load.
FIGS. 2-4 illustrate the schematic structure of general piezoelectric transformers.
FIG. 2 illustrates the schematic structure of a Rosen type piezoelectric transformer that is widely used for CCFL backlighting. A piezoelectric body of this piezoelectric transformer is in the form of a flat ceramic substrate which is wider than it is thick and is longer than it is wide. A pair of electrodes spaced apart in the thickness direction is formed on the piezoelectric body to cause polarization in the thickness direction. An electrode is also formed on a longitudinal end of the piezoelectric body to cause polarization in the longitudinal direction. When an input voltage Vin having a resonance frequency defined by the length of the piezoelectric body is applied to an input of the piezoelectric body, electrostriction causes strong mechanical vibrations of the piezoelectric body in the longitudinal direction, so that charges are produced on an oscillating portion Vout of the piezoelectric body due to piezoelectricity, thereby generating a high voltage output. Due to its high output impedance, the Rosen type piezoelectric transformer is suitable for igniting and lighting CCFLs. However, the Rosen type piezoelectric transformer has a low power transmission capacity, and its known maximum power is only 10 W.
FIG. 3 illustrates the schematic structure of a longitudinal vibration mode piezoelectric transformer that vibrates in the thickness direction.
This piezoelectric transformer includes a low impedance vibrating portion (input) including a plurality of piezoelectric layers and a high impedance vibrating portion (output) including a piezoelectric layer. The piezoelectric layers are laminated together, which cause vibrations in the longitudinal or thickness direction. The piezoelectric layers may be mechanically stressed when they are laminated together. This piezoelectric transformer is referred to as a “Transoner”, which has a high power transmission capacity, and its known maximum power is about 80 W. Although the longitudinal vibration mode piezoelectric transformer is efficiently used for step-up and step-down transformation, its output voltage is not high enough to drive CCFLs. Although the longitudinal vibration mode piezoelectric transformer can be used for a step-down AC-DC adapter (see U.S. Pat. No. 5,969,954), it is disadvantageous to ferrite converters since it still faces challenges in rectifying and smoothing the AC output voltage.
FIG. 4 illustrates the schematic structure of a ring-dot type piezoelectric transformer.
This piezoelectric transformer includes an input portion (specifically, a ring electrode) and an output portion (specifically, a dot electrode) that have the same polarization direction. This piezoelectric transformer is referred to as a “unipoled ring-dot type piezoelectric transformer”. The ring-dot type piezoelectric transformer is easier to manufacture than the Rosen type piezoelectric transformer of FIG. 2 and is advantageous in terms of power density. The ring-dot type piezoelectric transformer is also advantageous over the longitudinal vibration mode piezoelectric transformer of FIG. 3 in that it has a good impedance matching with CCFLs. In the ring-dot type piezoelectric transformer, an input voltage Vin applied across input electrodes on a vibrating portion having a low impedance is stepped up to a high output voltage Vout between output electrodes on an oscillating portion having a high impedance.
As is described above, the conventional power supply systems have low power efficiency and entail high manufacturing costs and are also large in size. To overcome these problems, studies have been made on a technology for removing the additional adapter, which is a major cause of the problems, to reduce the size of the power supply system and increase the power efficiency thereof.
One example is a technology for a CCFL power supply system (see U.S. Pat. No. 6,703,796) in which a DC-DC converter for stepping down the circuit drive voltage and a DC-AC inverter for stepping up the lamp drive voltage are integrated with a rectifier/filter circuit without an additional AC-DC adapter, thereby achieving a highly efficient power supply for LCD monitors. Especially, integrating the DC-AC inverter with the ferrite DC-DC converter in the power supply system is advantageous in terms of the efficiency, EMI noise, and size.